Polyphenylene ethers (also known as polyphenylene oxides) are a class of polymers widely used in industry, especially as engineering plastics in applications requiring toughness and heat resistance. For many such applications, the polyphenylene ethers are blended with various kinds of polystyrenes to improve their processability.
In recent years, it has become increasingly desirable to use polyphenylene ether/polystyrene resins in the form of sheets, molded or extruded articles and foamed sheets for food packaging as well as other applications. In food packaging applications, it is essential that the polyphenylene ether/polystyrene resin be substantially free from materials which are volatile, have undesirable odors or would otherwise harm the food. Various materials of this kind are known to be present in polyphenylene ether/polystyrene resins. They include odoriferous amines, particularly dialkylamines such as di-n-butylamine, which are components of the catalyst used in the preparation of polyphenylene ethers, and amines resulting from thermal cracking of polyphenylene ether-amine adducts. Also present may be by-products formed in the synthesis of the substituted phenols from which polyphenylene ethers are prepared. In the case of poly(2,6-dimethyl-1,4-phenylene ether) these frequently include 7-methyldihydrobenzofuran; 2,4,6-trimethylanisole (which is an especially powerful contributor to odor); 2,3-dihydrobenzofuran; 2,6-dimethylcyclohexanone and 2-ethylhex-2-enal. Styrene monomer is also a contributor to odor, as are toluene, ethyl benzene and other normally liquid aromatic hydrocarbon solvents.
Previous attempts to remove volatile compounds, including odoriferous species, from polyphenylene ether and/or polystyrene resins have been reported. However, none of them suggest unique suitability for making low odor polyphenylene ether/polystyrene pellets, solid sheets, molded or extruded articles and foamed materials. Aneja and Skilbeck, in commonly assigned EPO Patent Application No. 0226204, describe a method and apparatus for devolatilizing polymer solutions, including polyphenylene ether resins and polystyrene resins, in combination. The devolatilized polymer blend issues from the heat exchanger as a melt solution. Allen, Bacskai, Roberts and Bopp, in commonly assigned U.S. patent application Ser. No. 67,556, filed June 26, 1986, describe making low density high compressive strength foams from engineering polymer resins, e.g., polyphenylene ether resins and polystyrene resins, by feeding them to at least one extruder, adding a blowing agent and foaming the mixture through a die. The products of this process also do not exhibit a low odor because of the presence of volatile odoriferous species. Kasahara et al., U.S. Pat. No. 4,369,278, devolatilizes mixtures of polyphenylene ethers and rubber modified polystyrenes in a vacuum vented extruder with optional introduction of steam, but the content of volatile odoriferous substances, especially trimethylanisole, is not reduced sufficiently to meet the requirements for low odor foams for food packaging. Hasson and Pecak, in commonly assigned copending U.S. patent application, Ser. No. 07/291,534, filed Dec. 29, 1988, use a multi-stage extrusion process to devolatilize blends of polyphenylene ether resin and polystyrene resin using a solvent and a vacuum in each stage. However, although the source of odoriferous amine content is substantially reduced, further reduction is possible and other compounds such as 2,4,6-trimethylanisole remain, albeit at a greatly reduced level. Banevicius, in commonly assigned copending patent application Ser. No. 07/291,562, filed Dec. 29, 1988, now allowed describes a method to prepare solutions of polyphenylene ether in normally liquid aromatic hydrocarbons, such as toluene, having the trimethylanisole content reduced by use of clean reaction solvent or by use of distillation of the recycled reaction solvent to such low levels that they do not contribute odor to the ultimate compositions; however, the products still contain a substantial content of a source of volatile odoriferous amines.
In the present state of the art, therefore, a need still exists to provide low odor pellets, foamed sheets and boards, solid sheets, molded or extruded articles and the like comprising polyphenylene ether and polystyrene resins, preferably those having sufficient melt strength and thermal stability to make them microwavable and suitable for use in contact with food. Such a need is met by the process and articles of the present invention. Briefly, the process comprises blending a solution of polyphenylene ether resin having a low trimethylanisole content with a solution of styrene resin, devolatilizing the mixed solutions first in one or more heat exchange zones, then in a stripping zone, optionally with steam present, and then recovering the low odor resin in particulate form.